Using Sencha Touch with ASP.NET MVC will no (or very little) different to using it with ExtJS. All your doing is calling your MVC Actions from the Sencha Stores and /or Ajax calls just as you would with a static json file or a PHP file.

In your MVC Action you should make sure your returning JSON data which can be interpreted by the Store. Something like this will return some data in the correct format from your ASP.NET..

Since ASP.Net MVC 1 you have a type of Action named "JsonResult", instead of return an ActionResult on your method you should return a JsonResult. and with ASP.Net MVC you have a .ToJson() method in your objects wich make all the transformation you need.